Non-austemper treated spheroidal graphite cast iron

ABSTRACT

A non-austemper-treated spheroidal graphite cast iron obtainable without being subjected to an austemper treatment. The non-austemper-treated spheroidal graphite cast iron has a tensile strength of 650-850 MPa and an elongation of 7.0-14.5%. In the non-austemper-treated spheroidal graphite cast iron, V-notch material has a fatigue limit of 290 MPa or more. The non-austemper-treated spheroidal graphite cast iron has well-balanced mechanical properties both in tensile strength and elongation and improved tensile strength and elongation than conventional one.

TECHNICAL FIELD

The present invention relates to a non-austemper-treated spheroidalgraphite cast iron obtainable without being subjected to an austempertreatment.

BACKGROUND ART

As cast iron, there has been known spheroidal graphite cast iron inwhich graphite has a spheroidal shape. The spheroidal graphite cast ironhas a tensile strength within a range of from 400 to 800 MPa, and has atendency to decrease elongation when the tensile strength is increasedand to decrease tensile strength when elongation is increased.

Recently, in a field of automobile parts or the like, where lighteningis strongly required, spheroidal graphite cast iron having well-balancedmechanical properties both in tensile strength and elongation has beenrequired. As spheroidal graphite cast iron having such mechanicalproperties, the following spheroidal graphite cast iron has been known.

One is a bainite spheroidal graphite cast iron obtained by heating acasting at a temperature (about 800-950° C.) for austenitizing,quenching the casting in a salt bath furnace at about 300-400° C.,retaining the casting at a constant temperature in the furnace, andtaking out the casting from the furnace. Another example is a bainitespheroidal cast iron obtained by adding 1-4% by mass of Ni and 0.5-1.0%by mass of Mo thereto and not subjecting to a heat treatment under acondition of, so-called, as cast.

However, in the former bainite spheroidal graphite cast iron, asufficient bainite structure can not be attained in the case of heavythickness products. Therefore, it is used to be adopted for a thinproduct. Even in this case, there is caused a problem of strain due to athermal treatment, or a high cost due to a heat treatment using a saltbath furnace. The latter bainite spheroidal graphite cast iron has aproblem of cost-increase because expensive Mo is added thereto.

If the aforementioned bainite spheroidal graphite cast iron is subjectedto molten-zinc plating (for example, holding in zinc melt for 120seconds at 460° C.) to give corrosion resistance, the bainite spheroidalgraphite cast iron has a defect of decrease in tensile strength andelongation due to the heat treatment as shown in the following Table 1.

TABLE 1 Tensile Elonga- Organi- Thermal treatment, molten- strength(MPa) tion (%) zation zinc plating treatment 1150 12.0 Bainite Thermaltreatment only 850 4.0 Bainite Molten-zinc plating treat- ment afterthermal treatment

Table 1 shows influence of heat (about 460° C.) on spheroidal graphitecast iron having bainite structure. Here, “thermal treatment” meansholding at 900° C. for one hour and then at 380° C. for one hour, and“molten-zinc plating treatment” means holding at 460° C. for 120seconds.

Therefore, the present invention has been made in view of the aboveconventional problems, and an object of the present invention is toprovide a high strength and ductility spheroidal graphite cast ironhaving well-balanced mechanical properties both in tensile strength andelongation and having improved tensile strength and elongation thanconventional one.

Another object of the present invention is to provide spheroidalgraphite cast iron which is not decreased in mechanical properties evenif it is subjected to a treatment such as hot dipping and which hasimproved tensile strength and elongation without adding Mo thereto.

The other object of the present invention is to provide anon-austemper-treated spheroidal graphite cast iron obtained withoutbeing subjected to an austemper treatment where the material is heatedat a temperature for austenitizing, and then quenched at about 300-400°C., retaining the casting at a constant temperature.

DISCLOSURE OF INVENTION

According to the present invention, there is provided anon-austemper-treated spheroidal graphite cast iron obtainable withoutbeing subjected to an austemper treatment,

-   -   wherein the non-austemper-treated spheroidal graphite cast iron        has a tensile strength of 650-850 MPa and an elongation of        7.0-14.5%.

According to the present invention, there is further provided anon-austemper-treated spheroidal graphite cast iron obtainable withoutbeing subjected to an austemper treatment, wherein V-notch test piecehas a fatigue limit of 290 MPa or more.

The non-austemper-treated spheroidal cast iron preferably contains0.05-0.45% by mass of Mn, and in this case the non-austemper-treatedspheroidal cast iron preferably contains 2.0-4.0% by mass of Ni.

Further, the non-austemper-treated spheroidal cast iron preferably has aBrinell hardness of 230-285 HB and a flank wear of 0.13 mm or less in acutting distance of 1.7 km.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an explanatory view showing a shape of a cutting test piece.

FIG. 2 is an explanatory view showing a shape of a Y-shaped testmaterial (JIS B size).

FIG. 3 is an explanatory view showing a shape and a size of V-notchmaterial used for a rotary bending fatigue test.

FIG. 4 is a graph showing tensile properties (tensile strength, 0.2%proof stress and elongation).

FIG. 5 is a graph showing fatigue limit in Example 1.

FIG. 6 is a graph showing a relationship between hardness and tensilestrength/elongation.

FIG. 7 is an explanatory view showing a link of an electric powerproduct.

FIGS. 8(a) and 8(b) are graphs showing tensile properties (tensilestrength, 0.2% proof stress and elongation) before and after the platingtreatment. FIG. 8(a) is a graph showing tensile properties before theplating treatment, and FIG. 8(b) is a graph showing tensile propertiesafter the plating treatment.

FIG. 9 is an explanatory view showing a wheel-supporting part of anautomobile product.

FIG. 10 is a graph showing tensile properties (tensile strength, 0.2%proof stress and elongation) in Example 6.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention is hereinbelow described in detail.

The present invention relates to a high strength and ductilityspheroidal graphite cast iron obtainable without being subjected to anaustemper treatment which has conventionally been conducted.Specifically, the spheroidal graphite cast iron has a tensile strengthof 650-850 MPa and an elongation of 7.0-14.5%. The mechanical propertiesof both tensile strength and elongation are well balanced, and thetensile strength and the elongation are improved in comparison with aconventional spheroidal graphite cast iron.

Such a high strength and ductility non-austemper-treated spheroidalgraphite cast iron has larger tensile strength and elongation thanpredetermined values without being subjected to a heat treatment.Further, even if the cast iron is subjected to hot dipping or the like,mechanical properties thereof are not deteriorated.

A non-austemper-treated spheroidal graphite cast iron of the presentinvention has a tensile strength of 650-850 MPa, preferably 700-850 MPa,more preferably 750-850 MPa, and an elongation of 7.0-14.5%, preferably9.5-14.5%, more preferably 12.0-14.5%.

Here, mechanical properties of tensile strength and elongation ofspheroidal graphite cast iron were obtained according to a test methodprescribed by JIS Z 2201.

The aforementioned non-austemper-treated spheroidal graphite cast ironof the present invention preferably contains 0.05-0.45% by mass of Mn asa component, and more preferably 0.10-0.35% by mass of Mn. A correlationbetween tensile strength and elongation of the spheroidal graphite castiron can be controlled by changing an added amount of Mn. That is, if aMn content is decreased, the tensile strength is lowered, while theelongation is improved. On the other hand, if a Mn content is increased,the tensile strength is improved, while the elongation is decreased. Ifa Mn content exceeds 0.45% by mass, it becomes too hard, and theelongation falls below 7.0%. Incidentally, Mn inevitably gets mixed fromthe material or in a production process, and it is difficult in point ofpresent technique to lower the content below 0.05% by mass. As anothercomponent, 2.0-4.0% by mass of Ni is preferably contained. When a Nicontent is out of the above range, elongation tends to decrease.

Incidentally, the other components of a non-austemper-treated spheroidalgraphite cast iron of the present invention are not particularlylimited. However, it is preferable that the non-austemper-treatedspheroidal graphite cast iron includes 3.1-4.0% by mass of C, 1.8-3.0%by mass of Si, 0.05% by mass or less of P, 0.02% by mass or less of S,and 0.02-0.06% by mass of Mg. The reason is as follows:

-   (1) If the C content is below 3.1% by mass, carbide is formed and    elongation is remarkably reduced. If the C content is above 4.0% by    mass, carbon floatation causes deterioration in tensile strength.-   (2) If the Si content is below 1.8% by mass, carbide is formed and    elongation is remarkably reduced. If the Si content is above 3.0% by    mass, carbon floatation causes deterioration in tensile strength.-   (3) If the P content is above 5% by mass, a steadite phase is    formed, and it becomes brittle.-   (4) If the S content is above 0.02% by mass, MgS is formed upon a Mg    treatment, a dissolved Mg amount decreases to disturb spheroidizing    of graphite, and slag is increased. Therefore, it is not preferable.-   (5) If the Mg content is below 0.02% by mass, graphite cannot be    spheroide, and tensile strength cannot be ensured. If the Mg content    is above 0.06% by mass, carbide is prone to be formed, and a Mg    alloy upon a treatment is expensive. Therefore, it is not    preferable.

A non-austemper-treated spheroidal graphite cast iron has a property ofa fatigue limit of V-notch material of 290 MPa or more. It can beconsidered the fatigue limit becomes higher than predetermined leveleven in V-notch material because spheroidal graphite cast iron isparticularly excellent in an elongation property as described above.

Further, a non-austemper-treated spheroidal graphite cast iron of thepresent invention is excellent machinability. If a flank wear in thecase that a cutting test is performed is used as an index showingmachinability, spheroidal graphite cast iron of the present inventionhas a flank wear of 0.13 mm or less in a cutting distance of 1.7 km.

Cutting conditions for the cutting test were a cutting speed of 100m/min, transmission rate of 0.2 mm/rotation, and a cut of 1.5 mm to 10cutting test pieces having a shape shown in FIG. 1. A dry cutting wasperformed by the use of UC6010 produced by Mitsubishi Material as acutter.

Further, spheroidal graphite cast iron of the present invention has ahardness of 230-285 HB, preferable 235-280 HB, and more preferably240-275 HB with showing high hardness. Thus, spheroidal graphite castiron of the present invention has hardness above a predetermined one,and hardness is well balanced with strength and tenacity.

Here, the spheroidal graphite cast iron was measured for Brinellhardness by a method prescribed in JIS Z2245.

The aforementioned spheroidal graphite cast iron of the presentinvention may be produced by conventionally known steps.

An example of steps of producing cast iron is described. Various kindsof iron alloys such as pig iron and steel scrap from a material yard arecombined in consideration of contents of blended components to obtain amaterial. A cast iron molten metal is produced by using an electricfurnace (low-frequency furnace or high frequency furnace) or a cupola.The molten metal having an aimed composition is subjected to a moltenmetal treatment in a ladle using a graphite globurization agent. At thistime, inoculation may be added as necessary.

After the spheroidizing of the molten metal, the molten metal is pouredinto a mold molded by a molding machine from the ladle to be cast forsolidification and cooling as it is in the mold. Alter an article in themold is cooled, decomposition of the mold is performed by a shake-outmachine to separate the article from molding sand. The article is cooledby a drum cooler, and then sand adhering to the surface of the articleis removed by a shot blast to be subjected to fettling. In this fettlingstep, finishing such as a dam and deburring is performed to obtain aproduct of cast iron casting.

Among the above steps, in inoculation performed in a holding furnace anda molten metal treatment for spheroidizing, a desired spheroidalgraphite cast iron can be produced by adjusting kind and amount ofmaterials to be added thereto. In the present invention, a high strengthductile non-austemper-treated spheroidal graphite cast iron havingwell-balanced and high mechanical properties of tensile strength andelongation and in comparison with conventional ones can be obtained bycontrolling amounts of Mn and Ni to be predetermined ones as componentsand controlling cooling speed after a molten metal is poured into a moldin various methods except for a conventionally known austempertreatment.

That is, in the present invention, cooling speed is controlled afterspheroidal graphite cast iron molten metal prepared so as to have anaimed composition as a method. Modes for such a method are as follows:

(1) Representatively, natural cooling (as cast) is performed in a moldwhen a product has a thickness of about 25-50 mm.

(2) A thin product, for example, a product having a thickness of 10 mmor less is cooled down too quickly, and therefore, cast iron havingpredetermined mechanical properties as in the present invention can notbe obtained. Therefore, the cooling speed should be controlled bykeeping the mold warm (selecting a mold material which is hard to becooled, collecting a series of molds together, heating a mold, or thelike) to give almost the same cooling process for a product having athickness of about 25-50 mm.

(3) After the knock-out of the mold, cooling speed is controlled withheating the product to give almost the same cooling process for aproduct having a thickness of about 25-50 mm in the same manner as theabove (2).

To sum up, in a method of the present invention, cooling speed iscontrolled by gradually cooling continuously after casting or heatingafter cooling down to about a constant temperature after casting andthen cooling with heating without a quenching operation from atemperature for austenitizing to about 300-400° C. such as aconventionally known austemper treatment. In view of the difference incooling speed depending on a thickness of a product (A thin one iscooled quickly, while a thick one is cooled slowly.), cooling speed iscontrolled to obtain strong and tenacious spheroidal graphite cast ironhaving well-balanced mechanical properties of both tensile strength andelongation.

The present invention is hereinbelow described more specifically on thebasis of Examples.

EXAMPLE 1

In accordance with a conventionally known production steps of cast iron,molten metal of spheroidal graphite cast iron was prepared.

That is, cast iron materials were blended to prepare molten metal ofspheroidal graphite cast iron having an adjusted chemical composition of3.55% by mass of C, 2.50% by mass of Si, 0.29% by mass of Mn, 0.18% bymass of P, 0.07% by mass of S, 0.039% by mass of Mg, 0.036% by mass ofCr, 0.08% by mass of Cu, and 3.1% by mass of Ni.

The molten metal of spheroidal graphite cast iron was poured into a moldfor Y-shaped test material (JIS B size) 30 shown in FIG. 2 at about1400° C. and subjected to natural cooling (as cast) to a constanttemperature in the mold.

A test piece was taken from the lower portion 31 of the Y-shaped testmaterial (B size) 30 (JIS G 5502). Tensile properties (tensile strength,0.2% proof stress and elongation) were obtained by using No. 4 testpiece of JIS Z 2201. The results are shown in FIG. 4.

Further, a V-notch material 32 shown in FIG. 3 was taken from theY-shaped test material (B size) 30 and subjected to a rotary bendingfatigue test to obtain a fatigue limit.

In the rotary bending fatigue test, stress was applied with rotating theV-notch material 32 at 2500 rpm in the atmosphere at room temperatureusing the Ono-style rotary bending fatigue test-machine on the basis ofJIS Z 2274 so as to measure a fatigue limit from a correlation betweenstress and repeated number until the test piece was broken. The resultsare shown in FIG. 5.

EXAMPLE 2

A cutting test piece 10 of spheroidal graphite cast iron having a shapeshown in FIG. 1 was taken in the same manner as in Example 1. Thecutting test piece 10 was subjected to a cutting test to be measured forflank wear. The cutting test piece had a flank wear of 0.12 mm or lessin a cutting distance of 1.7 km.

On the other hand, a conventional spheroidal graphite cast iron(corresponding to FEC700) (composition: 3.6% by mass of C, 2.5% by massof Si, 0.4% by mass of Mn, 0.03% by mass of P, 0.03% by mass of S, 0.03%by mass of Mg, 0.8% by mass of Cu, and the rest of Fe) had a flank wearof 0.16 mm. Thus, it was found that spheroidal graphite cast iron of thepresent invention is excellent in workability.

EXAMPLE 3

A Y-shaped test material (B size) was obtained from spheroidal graphitecast iron molten metals having many various composition in ranges of0.05-0.45% by mass of Mn, 2.0-4.0% by mass of Ni, 3.1-4.0% by mass of C,1.8-3.0% by mass of Si, 0.05% by mass or less of P, 0.02% by mass orless of S, 0.02-0.06% by mass of Mg, and the rest of Fe and measured fortensile properties (tensile strength and elongation) in the same manneras in Example 1 and hardness. The results are shown in FIG. 6.

EXAMPLE 4

A link of an electric power product shown in FIG. 7 was measured fortensile properties (tensile strength, 0.2% proof stress and elongation)in the same manner as in Example 1. Test pieces were taken at the sitesof {circle around (1)}, {circle around (2)}, {circle around (3)},{circlearound (4)}, and {circle around (5)} of FIG. 7. The results are shown inFIG. 8(a).

EXAMPLE 5

The same link as in Example 4 was subjected to molten zinc plating (keptfor 120 sec. at 460° C.) and measured for tensile properties (tensilestrength, 0.2% proof stress and elongation). The results are shown inFIG. 8(b).

As a result, it was confirmed that tensile properties has almost nodifference between a like before plating and one after plating.

EXAMPLE 6

A wheel-supporting part of an automobile product shown in FIG. 9 wasmeasured for tensile properties (tensile strength, 0.2% proof stress andelongation). Test pieces were taken at the sites of A, B, C, D, and E ofFIG. 9. The results are shown in FIG. 10.

COMPARATIVE EXAMPLE 1

A Y-shaped test material (B size) was cast in the same manner as inExample 1 except for 0.53% by mass of Mn among molten metal compositionsof the spheroidal graphite cast iron, and test pieces were taken in thesame manner to be measured for tensile strength and elongation.

As a result, the elongation is lowered down to 6% or less though thetensile strength was increased to 850-900 MPa.

DISCUSSION

As is clear from the results of Examples 1, 4-6, and Comparative Example1, spheroidal graphite cast iron obtained in Examples 1, and 4-6 have atensile strength of 760-800 MPa, a 0.2% proof stress of 500 MPa or more,and an elongation of 7.0% or more and shows that it has expectedmechanical properties. In addition, there is obtained a high value of295 MPa of fatigue limit under a condition of 10⁷times of repetition ofV-notch material obtained in Example 1.

Further, as understood from Example 2, spheroidal graphite cast iron ofthe present invention is excellent in workability, has a predeterminedhardness of 230-285 HB and well-balanced mechanical properties inaddition to high strength and high tenacity.

Industrial Applicability

As described above, spheroidal graphite cast iron of the presentinvention can be obtained without being subjected to an austempertreatment. The spheroidal graphite cast iron has well-balancedmechanical properties both in tensile strength and elongation, and highstrength and high tenacity with tensile strength and elongation beingmore improved than conventional one. Further, spheroidal graphite castiron of the present invention does not deteriorate in mechanicalproperties even if it is subjected to hot dipping or the like, andtensile strength and elongation can be improved without Mo being addedtherein. Therefore, spheroidal graphite cast iron of the presentinvention can be preferably adapted to electric products such as linksor automobile parts such as wheel-supporting parts.

1. A non-austemper-treated spheroidal graphite cast iron obtainablewithout being subjected to an austemper treatment, characterized in thatthe non-austemper-treated spheroidal graphite cast iron has a tensilestrength of 650-850 MPa and an elongation of 7.0-14.5%, wherein thespheroidal graphite cast iron consists essentially of 0.05 to 0.45% bymass Mn, 2.0 to 4.0% by mass Ni, 3.1 to 4.0% by mass C, 1.8 to 3.0% bymass Si, up to 0.05% by mass P, up to 0.02% by mass S, 0.02 to 0.06% bymass Mg, and the balance Fe.
 2. A non-austemper-treated spheroidalgraphite cast iron obtained without being subjected to an austempertreatment according to claim 1, characterized in that V-notch materialhas a fatigue limit of 290 MPa or more.
 3. A non-austemper-treatedspheroidal graphite cast iron according to claim 1, wherein thenon-austemper-treated spheroidal graphite cast iron contains 0.1 to0.35% by mass of Mn.
 4. A non-austemper-treated spheroidal graphite castiron according to claim 1, wherein the non-austemper-treated spheroidalgraphite cast iron has a Brinell hardness of 230 to 285 HB.
 5. Anon-austemper-treated spheroidal graphite cast iron according to claim2, wherein the non-austemper-treated spheroidal graphite cast iron has aBrinell hardness of 230-285 HB.
 6. A non-austemper-treated spheroidalgraphite cast iron according to claim 3, wherein thenon-austemper-treated spheroidal graphite cast iron has a flank wear of0.13 mm or less in a cutting distance of 1.7 km.
 7. Anon-austemper-treated spheroidal graphite cast iron according to claim2, wherein the non-austemper-treated spheroidal graphite cast ironcontains 0.1 to 0.35% by mass of Mn.
 8. A non-austemper-treatedspheroidal graphite cast iron according to claim 7, wherein thenon-austemper-treated spheroidal graphite cast iron has a flake wear of0.13 mm or less in a cutting distance of 1.7 km.
 9. Anon-austemper-treated spheroidal graphite cast iron according to claim1, wherein the non-austemper-treated spheroidal graphite cast iron has aflank wear of 0.13 mm or less in a cutting distance of 1.7 km.
 10. Anon-austemper-treated spheroidal graphite cast iron according to claim2, wherein the non-austemper-treated spheroidal graphite cast iron has aflank wear of 0.13 mm or less in a cutting distance of 1.7 km.